RF broadcasting transmission apparatus for connecting high power transmitters to their antennas uses either coaxial line or waveguide as determined by factors such as frequency, power level, distance between transmitter and antenna, height of antenna tower, number of channels to be transmitted, and the like. For Very High Frequency (VHF) television, as for FM radio broadcasting and the various business and other bands embedded within the VHF range, the frequencies are low enough—which means the wavelengths are long enough and the structures must be large enough—to make waveguide-based transmission lines and signal manipulation largely infeasible. For the Ultra-High Frequency (UHF) television band, as for business broadcast channels with comparably high frequencies and frequencies higher still, up into the so-called microwave bands, waveguide may have utility comparable to or superior to that of coaxial line for many purposes.
Functions commonly performed at lower frequencies with discrete passive elements such as resistors, inductors, transformers, capacitors, transmission line sections, and the like can be replaced in waveguide systems by tuned cavities, dimension changes, resonant pins, blocks of solid dielectric material, and other apparatus to achieve comparable effects to the conventional components while working well at the power levels called for in RF transmission systems. An example of this, termed a waveguide-based switchless combiner, can accept two inputs, each of which is a broadcast signal from a transmitter. If the two signals come from transmitters that are synchronized, such as by accepting synchronous excitation and being well matched dimensionally, and if the frequency range for the switchless combiner includes the full channel width of the signals, then the switchless combiner can split each signal into two orthogonal parts, pass them through two waveguide sections, and join them into a single signal that can deliver virtually the full energy of the two transmitters to the output waveguide or coaxial line that carries it to the antenna, effectively adding the signal strength of two lower-power transmitters.
A transmitter system including a combiner device commonly requires one or more mechanical switches to direct signals from the transmitters to the combiner and/or from the combiner to either an antenna for broadcast or a resistive dissipative load device for test. Use of such mechanical switches generally requires shutting off power to the transmitters and may call for performing partial disassembly of high-power apparatus to reroute signals. A desirable capability would be to alleviate the need for one or more mechanical switches as well as to allow testing and maintenance functions to proceed without shutting off known-good transmitters and without the necessity of taking a programming source off the air altogether.
A type of hybrid known in the art as a “magic tee” or 180 degree hybrid differs from a standard or rectangular 90 degree hybrid in producing a substantially full-power output from an in-plane output port for two coherent inputs, and a substantially evenly split output between the in-plane and orthogonal output ports for two inputs out of phase by 90 degrees. Where the inputs have opposite phase, substantially all of the energy exits by the orthogonal port.
Accordingly, there is a need in the art for a switching system for broadcast transmission that overcomes, at least to some extent, the problems associated with the use of mechanical switches along with combiners to switch individual transmitters in and out of the broadcast signal stream.